Directional dual frequency antenna arrangement

ABSTRACT

Using two simple antennas with different frequencies of operation combined into an antenna arrangement a directional radiation pattern is obtained for both frequencies of operation. This is achieved by placing the antennas such that the first antenna acts as a director for the second antenna at the frequency of operation of the second antenna and the second antenna acts as a reflector for the first antenna at the frequency of operation of the first antenna. In the case of a monopole antenna the the first antenna is shorter than the second antenna and can thus operate as a director while the second antenna is longer than the first antenna and can thus act as a reflector.

The present invention relates to an antenna arrangement comprising afirst antenna element with a first operational frequency and a secondantenna element with a second operational frequency.

When designing a receiver or transmitter for dual frequency operation acommon choice of antenna arrangement comprises a first mono pole antennatuned for a first operating frequency and a second monopole antennatuned for a second operating frequency and selecting the antenna to beused depending on the choosen frequency of operation. When anotherfrequency of operation is choosen the associated antenna is selected andtransmission and reception accomplished through this antenna.

The monopole antenna is often choosen because of it's low cost.

The problem associated with this type of antenna arrangement is thatwith current communication standards directivity as well as dualfrequency operation is desired which would lead to a directional antennacomprising the radiating element as well as reflectors and/or directorsfor each operating frequency, thus leading to an antenna arrangementwith many antenna elements.

It is the object of this invention to provide a multi-frequencydirectional antennas with less antenna elements.

This objective is achieved in that the antenna arrangement according tothe invention is characterized in that the first antenna element is adirector for the second antenna and that the second antenna is areflector for the first antenna.

Antennas operated at higher frequencies are smaller than antennasoperated at lower frequencies. The size of the second antenna isdifferent from the size of the first antenna because of the differentoperational frequency. When the antenna arrangement is operated at thefirst operating frequency the first antenna acts as the radiatingelement of the antenna arrangement while the then passive secondantenna, because of its different size than the first antenna, acts aseither a director or a reflector for the first antenna. A directionalantenna arrangement is achieved this way.

When the antenna arrangement is operated at the second operatingfrequency the second antenna acts as the radiating element of theantenna arrangement while the then passive first antenna, because of itsdifferent size than the first antenna, acts as either a director or areflector for the second antenna. The size of the first antenna isdifferent from the size of the second antenna because of the differentoperational frequency.

When the first operational frequency is higher than the secondoperational frequency the first antenna element functions as a directorfor the second antenna element, while the second element functions as areflector for the first antenna element.

A further embodiment of the invention is characterized in that the firstantenna element is a mono-pole antenna. The mono-pole is a very simpleform of antenna that can function as the radiating element in an antennaand as a reflector or director and is thus especially suitable for usein the antenna arrangment according to the present invention.

A further embodiment is characterized in that the second antenna elementis a mono-pole antenna. The mono-pole is a very simple form of antennathat can function as the radiating element in an antenna and as areflector or director and is thus especially suitable for use in theantenna arrangment according to the present invention.

A further embodiment is characterized in that the first antenna elementis a quarter wavelength antenna element and the second antenna elementis a quarter wavelength antenna element. The quarter wavelengthmono-pole is an efficient radiator at an operational frequency where thelength of the monopole is a quarter wavelength of the operationalfrequency. In the present invention multiple antenna elements are used.When using quarter wavelength antenna elements a compact antennaarrangement is achieved.

A further embodiment is characterized in that the distance between thefirst antenna element and the second antenna element is approximatelyone quarter wavelength of the highest of the first and secondoperational frequency.

It was found that by placing the two antenna elements at a distance ofapproximately one quarter wavelength of the highest of the first andsecond operational frequency optimal reflection and direction wasobtained.

A Transceiver for dual frequency operation according to the inventioncomprising an antenna arrangement as claimed in claim 1, 2, 3 or 4yields a transceiver with an directional antenna suitable for dualfrequency operation.

A further embodiment of the transceiver is characterized in that thetransceiver comprises a second antenna arrangement identical to the nowfirst antenna arrangement. By including a second directional antennaarrangement the transceiver can better take advantage of thedirectionality of the antenna arrangement. Because of the low number ofantenna elements needed to provide two directional antenna arrangementsfor dual frequency operation it is possible to equip the transceiverwith two antenna arrangements.

A further embodiment of the transceiver is characterized in that thetransceiver is arranged to use the first antenna arrangement and thesecond antenna arrangement for antenna diversity. When two antennaarrangements are available to the transceiver the transceiver can useantenna diversity to obtain better transmission and reception. Becausethe antenna arrangements are directed in different directions and havedirectional transmission and reception characteristics the transceivercan for instance implement antenna diversity by selecting the strongestsignal coming from the two antenna arrangements thus improving thequality of reception and transmission.

A further embodiment of the transceiver is characterized in that thetransceiver is arranged to use the first antenna arrangement and thesecond antenna arrangement for beam steering. Since two directionalantenna arrangements can be used with the transceiver, because of theantenna arrangements compact size and directional characteristics, it ispossible to employ beamsteering in order to improve the transmission andreception. Any method for beamsteering using two directional antennaarrangements can be used to obtain the beamsteering.

A further embodiment of the transceiver is characterized in that thefirst antenna arrangement and the second antenna arrangement arearranged such that the antenna elements are all comprised in a plane,that the antenna elements of each antenna arrangement are parallel tothe other antenna elements in the antenna arrangement and that the firstantenna arrangement is placed at an angle between 20 and 60 degrees tothe second antenna arrangement.

By placing the antenna arrangements and the antenna elements in thisposition the theta components, ie.e horizontal polarization of theantenna, are directed in the horizontal plane.

To summarize: using two simple antennas with different frequencies ofoperation combined into an antenna arrangement, a directional radiationpattern is obtained for both frequencies of operation. This is achievedby placing the antennas such that the first antenna acts as a directorfor the second antenna at the frequency of operation of the secondantenna and the second antenna acts as a reflector for the first antennaat the frequency of operation of the first antenna. In the case of amonopole antenna the the first antenna is shorter than the secondantenna and can thus operate as a director while the second antenna islonger than the first antenna and can thus act as a reflector.

The invention will now be described based on figures.

FIG. 1 a shows an antenna arrangement comprising two mono-pole antennas.

FIG. 1 b shows an antenna arrangement comprising two mono-pole antennas.

FIG. 2 shows a communication device comprising two antenna arrangementswith two mono-pole antennas each.

FIG. 3 shows the radiation pattern of the antenna arrangement when thefirst antenna element acts as the radiating element.

FIG. 4 shows the radiation pattern of the antenna arrangement when thesecond antenna element acts as the radiating element.

FIG. 5 shows two antenna arrangements and the resulting radiationpattern as can be used for antenna diversity and for beam steering.

FIG. 6 shows the 3D radiation pattern of the antenna arrangement whenthe first antenna element acts as the radiating element.

FIG. 7 shows a transceiver with two antenna arrangements located in aplane and at an angle to each other.

In the FIGS. 1-3 the first antenna is drawn as a short monopole antennawhile the second antenna is drawn as a longer monopole antenna. Eventhough the figures are intended as schematic diagrams that normally notconfer information about physical proportions, the antennas are drawnsimilar to a normal configuration, i.e. one antenna shoerter than theother antenna and placed at a distance of each other which is comparableto the length of the short antenna. In FIG. 8 a physical arrangement ofthe antenna arrangement is shown.

FIG. 1 a shows communication station comprising an antenna arrangementcomprising two mono-pole antennas. In order to accommodate dualfrequency operation either a single dual frequency antenna or twoseparate single frequency antennas can be used. When using two antennasthe simplest form is the mono-pole antenna. In FIG. 1 a a communicationstation 1 with two monopole antennas 4,5 is shown where a singletransceiver 2 is used to transmit and receive signals through theantennas 4, 5. When operation at a particular frequency is required aswitch 3 connects the appropriate antenna 4, 5 to the transceiver 2.When one antenna is being used the other antenna is not being used. Thetransceiver 2 processes the signals. It transmits the data received atthe input 11 and makes the received data available at the output 14.

FIG. 1 a shows another implementation of a communication devicecomprising an antenna arrangement comprising two mono-pole antennas.Here the communication device 6 is required to operate at twofrequencies simultaneously. Each antenna 9,10 is therefore dedicated toa separate transceiver 7, 8. In FIG. 1 b antenna 9 is dedicated totransceiver 8 and antenna 10 is dedicated to transceiver 7. For thisreason no switching is required. The transceivers 7,8 have an input 13,12 and an output 16, 15.

In order for the first antenna 5 to act as a reflector or director ofthe second antenna 4 the first antenna 5 is required to be located at aparticular distance from the second antenna 4. It was found that adistance equal to one quarter of the wavelength of the operationfrequency of the antenna with the higher operation frequency is anappropriate distance when the antennas are each dimensioned to bequarter wave length antennas. In other words, the distance between theantennas 4,5 should be approximately the length of the shorter antenna 5of the two antennas 4,5.

It is also important to note that when switching the antennas 4, 5 withthe switch 3 the antenna not in use can be terminated with theappropriate impedance to ensure the antenna not in use acts as adirector or reflector as required. The termination impedance can beconnected to the antenna by a switch. By varying the terminationsimpedance the radiation characteristics of the active antenna can bealtered.

In FIG. 1 b where both antennas are operated simultaneously this isachieved differently. Both antennas operate at different frequencies.When the first antenna is operated at a first frequency, this firstfrequency is consequently not the operational frequency of the secondantenna. It is therefore possible to ensure that while being driven by atransceiver matched to the antenna at its operational frequency thesecond antenna is terminated at a different impedance at the operationalfrequency of the first antenna. The same applies to the first antenna.The first antenna can be driven by the transceiver where the transceiveris matched to the antenna at the operational frequency of the firstantenna while the termination impedance at the operational frequency ofthe second antenna can be choosen to ensure the first antenna acts as areflector or a director at the operational frequency of the secondantenna. In this way the communication device 6 can operate at bothfrequencies simultaneously and achieve directivity at each frequencywith just two monopole antennas without additional antenna elements.

Several methods can be employed to feed the antennas, for instancemicrostrip or strip line techniques.

FIG. 2 shows a communication device comprising two antenna arrangementscomprising two mono-pole antennas each. The communication device 20comprises a transceiver 21 which uses antenna diversity to obtain thebest reception quality. The switches 23, 24 allow the transceiver toselect two of a total of four antennas 25, 26, 27, 28. The first antenna26 and the fourth antenna 28 have the same first operational frequencywhile the second antenna 25 and the third antenna 27 have the samesecond operational frequency. The first antenna 26 and second antenna 25are physically gouped together, and the third anetnna 27 and the fourthantenna 28 are physically grouped together. The first antenna 26operates at a higher frequency as the second antenna 25 and forms,because of its shorter length than, and proximity to, the secondantenna, a director for the second antenna 25. The second antenna 25operates at a lower frequency than the first antenna 26 and forms,because of its longer length, and proximity to, the first antenna 26, areflector for the first antenna 26. The same is true for the combinationof the third antenna 27 and the fourth antenna 28 where the fourthantenna 28 acts as a director for the third antenna 27 and the thirdantenna 27 acts as a reflector for the fourth antenna 28.

The radiation pattern of each antenna 25, 26, 27, 28 is as a consequencedirectional. This directionality is a prerequisite for antennadiversity. If two antennas with identical reception of signals were tobe used for diversity it would not be possible to employ antennadiversity. In the configuration shown in FIG. 2 the first antenna 26 andthe second antenna 25 main lobe in the radiation pattern is towards theleft, while the main lobe of the radiation pattern of the third antenna27 and fourth antenna 28 is directed to the right. The transceiver 21can thus select two antennas 25, 26, 27, 28 based on operationalfrequency using the switches 23, 24 and then can select the antenna withthe better reception quality using well know techniques for antennadiversity.

In addition to antenna diversity, the configuration shown in FIG. 2 canalso be used for beam steering applications since also for beam steeringtwo directional antennas are advantageous. The phase differencesrequired for beam steering can be achieved by the transceiver in theusual fashion.

FIG. 3 shows a communication device using antenna diversity whileoperating at two frequencies simultaneously. The communication device 30comprises a first transceiver 31 and a second transceiver 32. The firsttransceiver is connected to the first antenna 34 and to the fourthantenna 33. The second transceiver is connected to the second antenna 35and the third antenna 36. The first antenna 34 and the fourth antenna 33operate at a first frequency which is higher than the operationalfrequency of the the second antenna 35 which is the same as theoperational frequency of the third antenna 36.

As explained in FIG. 1 b the first antenna 34 acts as a director for thesecond antenna 35 and the seoncd antenna 35 acts as a reflector for thefirst antenna 34. The fourth antenna 33 acts as a director for the thirdantenna 36 and the third antenna 36 acts as a reflector for the fourthantenna 33. If the antennas are physically located as shown in FIG. 3the main lobe of the radiation pattern of the first antenna 34 and thesecond antenna 35 is directed to the left and the main lobe of theradiation pattern of the third anetnna 36 and the fourth antenna 33 isdirected to the right. This provides the differences in signal receptionas desirable for antenna diversity.

As explained in the description of FIG. 1 b the first transceiver 31 ismatched to the antennas 35, 36 it is connected to at the operationalfrequencies of the antennas 35, 36, but must provide an appropriatetermination impedance at the operational frequency of the secondtransceiver 32 in er to turn the connected antennas 35, 36 intoreflectors for the antennas 33, 34 connected to the second transceiver32.

Vice versa the second transceiver 32 is matched to the antennas 33, 34it is connected to at the operational frequencies of the antennas 33,34, but must provide an appropriate termination impedance at theoperational frequency of the first transceiver 31 in er to turn theconnected antennas 33, 34 into directors for the antennas 35, 36connected to the first transceiver 31. This match outside theoperational frequency of the transceiver 31, 32 can be achieved bymodifying the impedance of the transceiver or by adding impedanceelements connected to the antennas. The added impedance elements arethen located outside the transceiver 31, 32.

In addition to antenna diversity, the configuration shown in FIG. 3 canalso be used for beam steering applications since also for beam steeringtwo directional antennas are advantageous. The phase differencesrequired for beam steering can be achieved by each transceiver in theusual fashion.

FIG. 4 shows the radiation pattern of the antenna operating at the firstfrequency in the antenna arrangement when the second antenna elementacts as a reflector. As can be seen the reflector causes theomnidirectiona radiation pattern of a regular monopole to be changedinto a direction pattern with a main lobe 40.

FIG. 5 shows the radiation pattern of the antenna operating at thesecond frequency in the antenna arrangement when the first antennaelement acts as a director. As can be seen the director causes theomnidirectiona radiation pattern of a regular monopole to be changedinto a direction pattern with a main lobe 50.

FIG. 6 shows two antenna arrangements and the resulting radiationpattern as can be used for antenna diversity and for beam steering. Byusing two antenna arrangements each pointed in a different direction theantenna patterns provide for differences in the reception as required byantenna diversity. When a signal is poorly received by one antennaarrangement because the main lobe of the antenna pattern is directedaway from the source, the main lobe of the other antenna arrangement islikely to be directed more advantageously.

FIG. 7 shows the 3D radiation pattern of the antenna arrangement whenthe first antenna element acts as the radiating element. Due to theabsence of grounding in front and behind the antenna arrangement asignificant part of the radiated energy is directed downwards. FIG. 6shows two main radiation directions. One main radiation direction isdirected to the upper-left side having mainly Phi components (verticalpolarization), the other to the lower left side having mainly Thetacomponents (horizontal polarization).

FIG. 8 shows a transceiver with two antenna arrangements 80, 81 locatedin a plane and at an angle to each other. The horizontal polarizationcan be directed in the horizontal plane by tilting the antennaarrangements 80, 81 so that the main radiation direction of thehorizontal polarization is lifted to the horizontal plane as is the casefor the configuration as shown in FIG. 7. The vertical polarization canbe positioned in the horizontal plane by tilting the antennaarrangements 80, 81 not towards each other as shown in FIG. 7 but awayfrom each other, thus bending the beam downwards. As can be seen in FIG.8 the ground plane 84 does not need to be perpendicular to the axis ofthe antenna 82, 83 but can be placed in the same plane as the axis ofthe antenna 82, 83.

1. Antenna arrangement comprising a first antenna element with a firstoperational frequency and a second antenna element with a secondoperational frequency characterized in that the first antenna element isa director for the second antenna and that the second antenna is areflector for the first antenna.
 2. Antenna arrangement as claimed inclaim 1 characterized in that the first antenna element is a mono-poleantenna.
 3. Antenna arrangement as claimed in claim 1 characterized inthat the second antenna element is a mono-pole antenna.
 4. Antennaarrangement as claimed in claim 1 characterized in that the firstantenna element is a mono-pole antenna having a quarter wavelengthantenna element and the second antenna element is a mono-pole antennahaving a quarter wavelength antenna element.
 5. Antenna arrangement asclaimed in claim 4, characterized in that the distance between the firstantenna element and the second antenna element is approximately onequarter wavelength of the first operational frequency.
 6. Transceivercomprising an antenna arrangement as claim
 1. 7. Transceiver as claimedin claim 6 characterized in that the transceiver comprises a secondantenna arrangement identical to the now first antenna arrangement. 8.Transceiver as claimed in claim 7 characterized in that the transceiveris arranged to use the first antenna arrangement and the second antennaarrangement for antenna diversity.
 9. Transceiver as claimed in claim 7characterized in that the transceiver is arranged to use the firstantenna arrangement and the second antenna arrangement for beamsteering.
 10. Transceiver as claimed in claim 6 characterized in thatthe first antenna arrangement and the second antenna arrangement arearranged such that the antenna elements are all comprised in a plane,that the antenna elements of each antenna arrangement are parallel tothe other antenna elements in the antenna arrangement and that the firstantenna arrangement is placed at an angle between 20 and 60 degrees tothe second antenna arrangement.